Function to check if the OS image parameter is valid
Function to check if the OS image parameter is valid

Add helper function to check if the OS image parameter contained in
the OS parameters of an opcode is valid.

Signed-off-by: Jose A. Lopes <0fb9c4f8511df4ec2ae28af879e9d462f6218a2c@google.com>
Reviewed-by: Hrvoje Ribicic <be5828fca31f89218048bec4189e30a581aa65da@google.com>

grnet/snf-ganeti,ganeti-github-testing/ganeti-test-1,leshchevds/ganeti,apyrgio/ganeti,andir/ganeti,ganeti/ganeti,onponomarev/ganeti,mbakke/ganeti,leshchevds/ganeti,mbakke/ganeti,bitemyapp/ganeti,bitemyapp/ganeti,apyrgio/ganeti,yiannist/ganeti,ganeti/ganeti,andir/ganeti,dimara/ganeti,yiannist/ganeti,grnet/snf-ganeti,leshchevds/ganeti,mbakke/ganeti,yiannist/ganeti,onponomarev/ganeti,ganeti/ganeti,andir/ganeti,ganeti-github-testing/ganeti-test-1,dimara/ganeti